Cnemidactis sp.

Blake, Daniel B. & Rozhnov, Sergei, 2007, Aspects of life mode among Ordovician asteroids: Implications of new specimens from Baltica, Acta Palaeontologica Polonica 52 (3), pp. 519-533 : 524-526

Cnemidactis sp.

Fig. 3 .

Geological horizon: Middle Ordovician (upper part of the Volkhov regional stage or the lower part of the Kunda regional stage, Upper Arenig).

Material.—PIN 4125/769 No disk elements are available. Arm fragment 31 mm in length, 7 mm in breadth proximally, 5 mm in breadth distally, exposed in ventral view. A slightly shorter fragment of a more distal? arm interval is adjacent to the larger fragment. The ventral surface of the second fragment is also exposed. Preservation is quite good, but only ambulacrals, adambulacrals, and a single series of apparent marginals immediately adjacent to the adambulacrals are exposed. Ossicular form indicates a single species and size and close positioning suggest but do not prove a single individual. Specimens were collected at an abandoned limestone quarry located on the left side of the Volkhov River to the south of the village of Izvoz, northwestern Russia (St−Petersburg region). In this quarry, a clay limestone spans the upper part of the Volkhov regional stage and the lower part of the Kunda regional stage, both Upper Arenig; scattered ossicles have been found only in the Upper Volkhov portion of the interval, and it is likely the more complete fragments were collected from this portion of the section as well.

Description.—Arms elongate, cylindrical; disk unknown.

Abactinals, marginals.—Marginals in a single series ( Fig. 3B, F, G ), these above and abutting adambulacrals. Ossicles dorsal to enlarged marginals disrupted but not enlarged nor differentiated as to suggest a second marginal series. Marginals alternate with adambulacrals, resting in re−entrants formed by successive adambulacrals. Marginals approximately 2 mm in breadth. Marginals robust, closely abutted, paxilliform; column short, base robust, angular, sides of bases extend between adambulacrals, sides flattened forming angular ossicular margin. Exposed surface of marginals flat to weakly convex, closely pustulate. Spinelets small, fine, uniform, cylindrical, scattered along adambulacral−marginal boundary, spinelets might be derived from either or both ossicular types. Two incompletely exposed ossicles similar to but smaller than marginals appear to be abactinals ( Fig. 3G ).

Ambulacrals.—Ambulacral ( Fig. 3A, C, D ) ossicles nearly paired across arm axis; local offset might be taphonomic. Ambulacrals approximately square in outline; longitudinal canal deep, consisting of narrow axial canal bordered by a sloping surface formed by adradial margin of transverse ridge. Transverse ridge sharp, J−shaped, adradial flange (the base of the “J”) robust, transverse canal crosses ridge distally. Podial basin broad, complete, no indication of podial pore; contact between successive ossicles sinuous, approximately medial in basin, abradial flange of more distal basin extending over distal rim of the more proximal basin. Abradial end of ambulacral weakly overlaps adambulacral. Abactinal surface of the ambulacrals unknown.

Adambulacrals.—Twenty−one adambulacrals ( Fig. 3A–G ) present along 31 mm fragment; ossicular width approx. 3.5 mm. Adambulacrals upright in arm, wide, approximately rectangular in outline, abradial margin rounded; nose prominent, abutting abradial base of ambulacral. Sides of nose rounded, forming podial walls. Outer face weakly arched, with numerous similar, closely spaced pustules; pustules along proximal, distal margins of adambulacrals slightly smaller than uniform pustules over remainder of surface. Actinal surface of nose massive, pustules not clearly developed. Ossicular side faces ( Fig. 3E ) with elliptical interadambulacral articulation surfaces consisting of a marginal rim enclosing a depressed surface. Fine spinelets remain near adambulacrals, marginals.

Other ossicular types not exposed.

Remarks.—Assignment to Cnemidactis is based on presence of interdigitated adambulacrals and marginals, the marginals quite robust rather than truly paxilliform, and the presence of a uniform series of small spine bases on both ossicular types ( Spencer 1918). The comparatively small marginals of Cnemidactis sp. are unlike those of the type ( Spencer 1918: pl. 13: 2) but perhaps suggestive of those of C. osloensis Hansen, Bruton, and Jacobsen, 2005 ; because of absence of data on the mouth frame of the present specimen, no species assignment is made. Another occurrence of Cnemidactis is cited by Blake and Guensburg (1993).

Figures

Fig. 3. Urasterellid asteroid Cnemidactis sp., PIN 4125/769, Middle Ordovician, Volkov, Russia. A. Entire specimen, ventral view. B. Lower right portion of A, rotated; adambulacral series at top with nose directed upward and overlapping ambulacrals; taphonomically displaced marginal series in middle showing lateral facets (below), central waist (upper arrow), and crown. Adambulacrals of the second fragment are below, scale is to right of an adambulacral nose, lower arrow points to nose of rotated adambulacral; robust interadambulacral articular surfaces at lower left. C. Right portion of A; ambulacrals medial, series of spine pustules on nose of adambulacrals (upper arrow), two spines have fallen into the basin below the arrow; triangular articular surface of ambulacral (lower arrow) is equivalent to that illustrated by Spencer (1918; Fig.6A herein), articular flanges also visible on adjacent ambulacrals. D. Displaced ambulacrals medially, with adambulacrals; triangular articular surface on ambulacral (arrow). E. Marginal in lateral view (upper arrow), and adambulacral series, the lower arrow at inter−adambulacral articular surfaces. F. Adambulacral series above, marginal series below, these fitted between successive adambulacrals, the relationship indicating Cnemidactis. G. Ossicles from second fragment, lower center of A; series of disrupted marginals (arrows), abactinals below right marginal. Adambulacrals are above marginals; edge of adambulacral series of primary fragment is below. Scale bars: A, 3 mm; all others, 1 mm.